It's a Fish-Eat-Fish World
Some 300,000 marine species are known to science—about 15 percent of all the species identified on the planet. But the sea is so vast that a million or more as yet unknown species may live in its waters. Most of these aquatic species are tied together through the food web.
Level One: Photoautotrophs
The foundation of the sea's food chain is largely invisible. Countless billions of one-celled organisms, called phytoplankton, saturate sunlit upper-ocean waters worldwide. These tiny plants and bacteria capture the sun's energy and, through photosynthesis, convert nutrients and carbon dioxide into organic compounds. On the coast, seaweed and seagrasses do the same thing.
Together, these humble plants play a large role: They are the primary producers of the organic carbon that all animals in the ocean food web need to survive. They also produce more than half of the oxygen that we breathe on Earth.
Level Two: Herbivores
The next level of the marine food chain is made up of animals that feast on the sea's abundant plant life. On the ocean's surface waters, microscopic animals—zooplankton, which include jellyfish and the larval stages of some fish, barnacles, and mollusks—drift across the sea, grazing opportunistically. Larger herbivores include surgeonfish, parrotfish, green turtles, and manatees.
Despite their differences in size, herbivores share a voracious appetite for ocean vegetation. Many of them also share the same fate—which is to become food for the carnivorous animals of the food chain's top two levels.
Level Three: Carnivores
The zooplankton of level two sustain a large and diverse group of small carnivores, such as sardines, herring, and menhaden. This level of the food chain also includes larger animals, such as octopuses (which feed on crabs and lobsters) and many fish (which feed on small invertebrates that live near shore). Though these animals are very successful hunters, they often fall prey to a simple fact of ocean life: big fish eat smaller fish.
Level Four: Top Predators
The large predators that sit atop the marine food chain are a diverse group that includes finned (sharks, tuna, dolphins), feathered (pelicans, penguins), and flippered (seals, walruses) animals. These apex predators tend to be large, fast, and very good at catching prey. They are also long-lived and usually reproduce slowly.
But the marine food chain's top predators are common prey for the most deadly hunters of all—humans. When top predator species are depleted, their numbers are often slow to rebound, and their loss can send shock waves through the entire food web.
Alternative Food Chains
The primary marine food web, which is based on plant productivity, includes many of the sea's species—but not all of them. There are other deep-ocean ecosystems that are entirely independent of the sunlight energy that kick-starts the main marine ecosystem. At their roots, these unique ecosystems are fuelled by chemical energy, which enters the ocean from sources like seafloor hydrothermal vents.
Find out how we can balance our tastes with what's right for the oceans.
With today's technology, the fish you pull from your freezer is delicious, nutritious, more economical, and often better for the environment—and fishermen—than fresh-caught seafood.
High-tech harvesting and wasteful management have brought world fish stocks to dangerous lows.
Sustainable seafood advocate, chef, writer, and speaker Barton Seaver uses his talent and passion to inspire change in the management of our oceans and the communities who rely upon their bounty.
Ocean Photo Galleries
- Ballard, Robert
- Bowermaster, Jon
- De Rothschild, David
- Doubilet, David
- Earle, Sylvia
- Frozen Seafood Benefits
- Goodman, Beverly
- Habitat Destruction
- Invasive Species
- Kristof, Emory
- Marine Food Chain
- Marine Pollution
- Nicklen, Paul
- Norman, Brad
- Ocean Overview
- Pristine Seas Expeditions
- Sala, Enric
- Seafood Decision Guide
- Seafood Substitutions
- Sea Level Rise
- Sea Temperature Rise
- Seaver, Barton
- Sustainable Seafood
- Thys, Tierney
- Tips to Save the Ocean
National Geographic Magazine
A cast of thousands clings to rocky real estate in a narrow strip of shore called the intertidal zone.
From tiny coral polyps grew a marvel: Australia’s Great Barrier Reef. Could it all come crumbling down?
Carbon dioxide we pump into the air is seeping into the ocean and slowly acidifying it.
Support the Ocean
Help protect the last healthy, undisturbed places in the ocean so we can learn how to help healthy reefs thrive, help unhealthy reefs recover, and better preserve the ocean.
Explore the Ocean
Order ocean books, DVDs, maps, and more from the National Geographic online store.
Explore the world's oceans, from their prehistoric beginnings to modern-day efforts to preserve their natural wonder.
Immerse yourself in the wonders of the deep through colorful maps, photos, and satellite images.
Engage, Conserve, Restore
The National Geographic Society’s freshwater initiative is a multi-year global effort to inspire and empower individuals and communities to conserve freshwater and preserve the extraordinary diversity of life that rivers, lakes, and wetlands sustain.